STRUCTURAL LINKS BETWEEN ZEOLITE-TYPE AND CLATHRATE HYDRATE-TYPE MATERIALS - SYNTHESIS AND CRYSTAL-STRUCTURE OF [N(CH3)4]6[ALXSI8-XO18-X(OH)2-4)(X] . 44H2O (X = 3)

A novel tetramethylammonium aluminosilicate hydrate with the approxima te composition [NMe4]6[AlxSi8-xO18-x(OH)2+x] . 44H2O (x = 3-4) has bee n identified by powder X-ray diffraction as a component in a polyphasi c solid mixture which crystallized at room temperature from an aqueous NMe4OH-Al2O3 -SiO2 solution. Large crystals of the novel hydrate phas e could be mechanically selected from that mixture. The crystal struct ure has been determined from 1196 unique MoKalphaBAR diffraction data measured at 180 K: Tetragonal crystal system, cell constants a = 16.18 1(4) and c = 17.450(4) angstrom, space group P4/mnc with Z = 2 formula units per unit cell, R = 0.072. The host-guest compound is of polyhed ral clathrate type with a mixed three-dimensional, (mainly) four-conne cted network composed of oligomeric aluminosilicate anions [AlxSi8-xO1 8-x(OH)2+x]6- and H2O molecules linked via hydrogen bonds O-H ... O. T he aluminosilicate anions possess a cube-shaped (double four-ring) str ucture. Orientationally disordered cationic guest species NMe4+ are en closed in the large [4(6)6(8)] and [4(1)5(10)06(7)] polyhedral voids o f the host framework; the small [46] cages (i.e. the double four-ring anions) and [4(3)5(6)] cages are empty. The hydrate is a further membe r in a recently discovered series of clathrates with mixed tetrahedral networks, which provides a structure-chemical link between zeolite- a nd clathrate hydrate-type host-guest compounds.